The present invention relates to a process for preparing hydroformylation products of olefins having at least four carbon atoms, in which a high proportion of both the linear Ci-olefins having a terminal double bond comprised in the olefin-comprising feed used and of the linear Ci-olefins having an internal double bond is converted into hydroformylation products. Furthermore, the invention relates to a process for preparing 2-propylheptanol which comprises such a hydroformylation process.
Hydroformylation or the oxo process is an important industrial process and is employed for preparing aldehydes from olefins, carbon monoxide and hydrogen. These aldehydes can, if appropriate, be hydrogenated by means of hydrogen to give the corresponding oxo alcohols in the same process. The reaction itself is strongly exothermic and generally proceeds under superatmospheric pressure and at elevated temperatures in the presence of catalysts. Catalysts used are Co, Rh, Ir, Ru, Pd or Pt compounds or complexes which can be modified with N- or P-comprising ligands to influence the activity and/or selectivity. In the case of the hydroformylation reaction of olefins having more than two carbon atoms, mixtures of isomeric aldehydes can be formed as a result of the possible addition of Co to each of the two carbon atoms of a double bond. In addition, when olefins having at least four carbon atoms are used, double bond isomerization, i.e. shifting of internal double bonds to a terminal position and vice versa, can also occur.
Owing to the significantly greater industrial importance of the α-aldehydes, optimization of the hydroformylation process so as to achieve very high conversions combined with a very low tendency to form olefins having double bonds which are not in the α position is sought. In addition, there is a need for hydroformylation processes which, even starting out from internal or linear olefins, lead to α- and in particular n-aldehydes in good yields. Here, the catalyst used has to make both the establishment of an equilibrium between internal and terminal double bond isomers and the very selective hydroformylation of the terminal olefins possible.
Thus, for example, there is a need for plasticizer alcohols having from about 6 to 12 carbon atoms and a low degree of branching (known as semilinear alcohols) and corresponding mixtures thereof for the production of ester plasticizers having good use properties. These include, in particular, 2-propylheptanol and alcohol mixtures comprising this. They can be prepared, for example, by subjecting C4-hydrocarbon mixtures comprising butenes or butenes and butanes to hydroformylation and subsequent aldol condensation. When hydroformylation catalysts having an insufficient n selectivity are used, the hydroformylation can easily result in formation of not only n-valeraldehyde but also undesirable product aldehydes, which adversely affects the economics of the entire process.
The use of phosphorus-comprising ligands for stabilizing and/or activating the catalyst metal in rhodium-catalyzed low-pressure hydroformylation is known. Suitable phosphorus-comprising ligands are, for example, phosphines, phosphinites, phosphonites, phosphites, phosphoramidites, phospholes and phosphobenzenes. The most widespread ligands at present are triarylphosphines such as triphenylphosphine and sulfonated triphenylphosphine since these have a satisfactory activity and stability under the reaction conditions. However, these ligands have the disadvantage that they generally give satisfactory yields, in particular of linear aldehydes, only in the presence of very large excesses of ligand and internal olefins are not reacted to any appreciable extent.
On the other hand, it has been reported that particular catalysts make hydroformylation of linear olefins with increased selectivity to unbranched reaction products possible. Thus, U.S. Pat. Nos. 4,668,651, 4,748,261, 4,769,498 and 4,885,401 disclose particular rhodium/bisphosphite catalysts which allow hydroformylation of various linear olefins, e.g. propylene, butenes and hexenes, with sometimes good selectivity to unbranched reaction products. The conversion of olefins having an internal double bond into linear hydroformylation products can sometimes also be successfully carried out using the rhodium/bisphosphite catalysts described there.
J. Kolena, P. Morávek, J. Lederer, DGMK Tagungsbericht (2001), 2001-4 (Proceedings of the DGMK Conference “Creating Value from Light Olefins—Production and Conversion”, 2001), 119-126, it is mentioned on page 121 that rhodium/bisphosphite catalysts as are described in the abovementioned US patent texts have been used in Union Carbide's UNOXOL 10 process for preparing 2-propylheptanol from raffinate II.
A similar process for preparing 2-propylheptanol using 2-butene is described in the patent application WO 03/018192, where chelating phosphordiamidites are used as cocatalysts.
The abovementioned processes using specific rhodium/bisphosphite catalysts have the advantage of partial utilization of olefins having an internal double bond, but the phosphite ligands used or their derivatives have the disadvantage that they undergo various degradation reactions under customary hydroformylation and/or distillation conditions. These include, for example, hydrolysis, alcoholysis, transesterification, Arbusov rearrangement and reaction with cleavage of O—C and P—O bonds, as are described in P. W. N. M. van Leeuwen, Appl. Cat. A: General 2001, 212, 61.
In the process described in U.S. Pat. No. 4,426,542, the hydroformylation is carried out using cobalt catalyst under high-pressure conditions, as a result of which utilization of olefins having an internal double bond is likewise made possible. However, the proportion of n compounds in the hydroformylation products obtained is comparatively low. In addition, the process comprises a step which is carried out under high pressure. The capital costs for high-pressure processes are significantly higher than for low-pressure processes, so that the process is economically disadvantaged.
To achieve virtually complete utilization of olefins in olefin mixtures such as raffinate II using stable rhodium/phosphane catalysts, the process variant described in WO 01/55065 A1 has been developed. This describes a process for the integrated preparation of C9-alcohols and C10-alcohols from raffinate II, in which the butenes comprised in the raffinate II are largely utilized in the hydroformylation step, However, only the α-olefin 1-butene in the raffinate II is utilized for producing the C10-alcohol by means of aldol condensation and hydrogenation. Utilization of the 2-butene is achieved only by means of unavoidable coproduction of C9-alcohols.
If the hydroformylation is carried out as a single-stage process, complete or virtually complete conversions of the olefins used into preferably linear hydroformylation products can therefore frequently not be realized for technical reasons or reasons of process economics. This applies particularly to the use of olefin mixtures which comprise olefins of differing reactivity, for example olefins having internal double bonds and olefins having terminal double bonds. Processes in which the hydroformylation is carried out in two or more reaction stages have therefore been developed. Here, the reactors are, for example, in the form of a cascade in which the individual reactors are operated under different reaction conditions. In this way, it is possible, at a given reaction volume, to achieve a higher conversion than in an individual reactor of the same volume. Thus, for example, DE-A-100 35 120 and DE-A-100 35 370 describe processes for the hydroformylation of olefins in a two-stage reaction system.
EP-A-0 562 451 and EP-A-0 646 563 describe processes for preparing mixtures of isomeric decyl alcohols by two-stage hydroformylation of an olefin mixture comprising 1-butene and 2-butene, aldol condensation of the resulting aldehyde mixture and subsequent hydrogenation. In the process described in EP-A-0 562 451, the first stage predominantly converts 1-butene into valeraldehyde with an n selectivity of greater than 90%, while the unreacted olefins, predominantly 2-butene, are converted into n- and i-valeraldehyde in the second reaction stage. The second stage gives a valeraldehyde having a comparatively low proportion of the n compound. The total proportion of n compounds is thus significantly less than 90%. In addition, the process comprises a step which is carried out at high pressure. The capital costs for high-pressure processes are significantly higher than for low-pressure processes, so that the process is economically disadvantaged.
It is generally known that the isomerization of 2-butenes to 1-butene is an equilibrium reaction. Cis-2-butene, trans-2-butene and 1-butene are present in equilibrium with one another. The thermodynamic data are presented in D. Stull, “The Chemical Thermodynamics of Organic Compounds”, J. Wiley, New York 1969. An appropriate combination of isomerization and hydroformylation enabled the utilization possibilities for olefins having an internal double bond and for olefin mixtures comprising such olefins to be considerably improved.
Thus, Beller et al. in Chem. Eur. J. 5 (1999), 1301-1305, describe a process in which an isomerization step and a hydroformylation step are carried out in parallel. Here, two different homogeneous catalyst systems are used in one reactor. One of these catalyzes the isomerization and the other catalyzes the hydroformylation. A disadvantage of this process is that the two catalysts have to be matched to one another in a complicated fashion.
A process sequence in which the isomerization step and the hydroformylation step are carried out separately therefore comes into consideration. Although the double bond isomerization of olefins is known per se, specific requirements have to be taken into account in an industrial reaction which requires efficient coupling with a hydroformylation stage.
For example, U.S. Pat. No. 4,409,418 teaches that internal olefins can be isomerized to terminal olefins over Zr phosphates which are doped with Cr and/or Th.
It is known from EP-A-751 106 that 1-butene can be obtained from a C4-hydrocarbon stream by subjecting the C4-hydrocarbon stream to a selective hydrogenation and a fractional distillation, separating off a pure 1-butene fraction and then separating off the paraffins from the remaining 2-butene-comprising fraction by means of a molecular sieve and subjecting the resulting olefin-comprising stream to a double bond isomerization and recirculating it to the selective hydrogenation. A disadvantage of this process is that the fraction which has been subjected to the isomerization is recirculated to the hydrogenation step instead of directly to the distillation step. As a result, the volume of the circulating stream is inflated and the reactor in which the hydrogenation is carried out is burdened to a high degree with compounds which are inert toward the hydrogenation and are removed only in the subsequent distillation.
WO 02/096843 describes a process for obtaining 1-butene from 2-butenes. Here, a hydrocarbon stream comprising mainly 2-butenes is subjected to an isomerization and the reaction mixture formed is subjected to a distillation. In the distillation, a 1-butene-rich stream is separated off from a 2-butene-rich stream and the latter is recirculated to the isomerization step. However, this process is uneconomical for a hydrocarbon stream which comprises significant amounts of 1-butene. As a result of the distillation being carried out after the isomerization step, interfering volatile constituents of the feed (e.g. alkynes) can get into the isomerization reactor and there damage the catalyst or lead to formation of undesirable by-products.